For producing polyolefins using highly active catalysts, gas-phase polymerization processes, particularly those using a fluidized-bed polymerizer, are now extensively employed because they eliminate the step of removing spent catalyst and low-molecular weight polymer by-product.
FIG. 6 shows a typical fluidized-bed reactor for the polymerization of olefins. The reactor 7 is equipped with a gas distribution plate 2 through which olefin gas or an olefin-containing feed gas is supplied uniformly into a fluidized bed 3. This gas is introduced into the reactor through a suction pipe 4 at the bottom of the reactor; distributed uniformly by means of the distribution plate 2; and bubbles up through the bed of catalytic or reacting solids, thus fluidizing the bed to initiate and continue the polymerization reaction. The unreacted gases ascend through the fluidized bed 3 and are exhausted at a discharge pipe 5 atop the reactor, cooled and returned to the reactor via the suction pipe 4 for subsequent use.
If the feed gas is not uniformly distributed by the distribution plate 2, uniform agitation and mixing is not achieved in the fluidized bed 3 and the heat of polymerization is not completely removed from the fluidized bed. This is particularly a problem at the corners 6 defined by the inner surface of the side wall of the reactor and the distribution plate 2. Such non-uniform agitation can cause the formation of polymer aggregates and the deposition of polymer on the inside surface of the reactor, which alternately could lead to shutdown of the operation or deterioration in the quality of the product.
While perforated plates are commonly used as gas distribution plates, many proposals have been suggested to overcome the problem of non-uniform distribution by providing various types of caps above the porous plate. For example, JP-A-58-154702 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposed a trigonal pyramidal cap; U.S. Pat. No. 4,521,378, a cap using partition walls; European Patent Application 89691A, a bubble cap; and European Patent Application 173261A, an angle cap.
A gas distribution plate having small holes has a problem in that even limited deposition of polymer particles can cause a blockage and greatly reduce the flow of gas passing through the holes in the plate.
Further, when unreacted gases that often contain fine polymer powder are exhausted through the discharge pipe 5 and recycled into the reactor, a portion of this fine polymer powder will be trapped by the gas distribution plate and block the upward passage of gas. If the gas distribution plate plugs up in this way, the polymerizer cannot be continuously operated for a prolonged period of time.
This blockage problem can be partly solved by using a gas distribution plate having large holes. But this solution creates additional problems. First, polymer particles will drop through the distribution plate and onto on the inside surface of the reactor below the plate where they can form polymer aggregates. Second, the distance between adjacent holes, or their pitch, must be extended to create low-fluidity areas between holes. In addition, in order to prevent blockage of the distribution plate and the dropping of polymer particles through it, the flow rate of the feed gas per hole must be maintained higher than a certain value.
The balance between these factors is difficult to achieve. If the flow per hole of a feed gas supplied into the polymerizer for a given total amount is increased by increasing the hole diameter, for example, the total number of holes in the distribution plate must be reduced, whereby the pitch of holes is extended. If the pitch of holes in a simple perforated plate which is adapted to blow gases upward in a vertical direction is increased, polymer particles in the intermediate area between adjacent holes are not blown off the plate by the ascending gas and instead are deposited and can form polymer aggregates.
With a view to solving these two problems, many proposals have been made that rely upon providing a cap above each of the holes in the gas distribution plate. However, the caps themselves impede the fluidization of solids just above the distribution plate, thereby increasing the likelihood of difficulties such as deposition and agglomeration of polymer particles.
Irrespective of the hole diameter, the corners of the fluidized bed defined by the inside surface of the side wall of the reactor and the gas distribution plate present a significant resistance to the fluidization of solids. In order to prevent the occurrence of problems of the types described above, it is necessary to maintain effective fluidization in these areas.
The pressure difference that develops across the gas distribution plate in a common fluidized-bed gas-phase polymerizer is in the range of 200 to 10,000 mmH.sub.2 O and the diameter of the polymerizer is usually in the range of 1 to 5 m. Given the pressure difference and diameter in these ranges, the gas distribution plate, if it is a single solid plate, must have a thickness of from several to 50 millimeters in order to insure sufficient strength.
If the gas distribution plate is divided into segments each of which is reinforced with a rib support or some other suitable means, sufficient strength is insured even if the plate has a small thickness. However, the rib supports impede the fluidization of solids and increase the chance of the occurrence of such troubles as the deposition and agglomeration of polymer particles.